Signal selecting circuit



Oct. 30, 1934. H. J. LOFTIS' 1,978,466

SIGNAL SELECTING G IRCUIT Filed May 51, 1932 2] wu c/wbo' HDMER .J Lara5 Patented Get. 30, 1934 rarest" QFFECE 1,978,486 SIGNAL SELECTINGCIRCUIT Homer J. Loftis, Ironton Ohio, assignor, by mesne assignments,to Radio Corporation of America, New York, N. Y., a corporation ofDelaware Application May 31, 1932, Serial No. 614,477 10 Claims. (01.250-20) This invention relates to a selective circuit, and

more particularly toa signal selecting circuit.

Inasmuch as it is the purpose of a preselecting circuit to eliminate, asfar as possible, all signals except the one that it is desired toreceive, and thus aid in the selection of signals, it is an object ofthis invention to provide a preselector circuit of improved efiiciencywhich will more effectively aid in the'selection of signals than thosecommonly in use.

Another object of this invention is to provide a structure which, whileproviding selective emciency, will also permit good energy transferefficiency for that type of circuit.

Another object of this invention is to provide a structure which issimple in construction and consequently inexpensive to build.

Still another object of the present invention is to provide a signalselecting circuit having flexibility of design.

Further objects and advantages of the present invention will beapparentfrom the following description, reference being had to theaccompanying drawing wherein several forms of the present invention areclearly shown.

In the drawing: i

Fig. l discloses a form of signal selecting circuit utilized as apreselcctor circuit to couple an antenna and ground to the input of aradio receiver.

Fig.2 discloses a modificationof the circuit, utilized for the samepurpose asthat in Fig. 1.

With particular reference to Fig. 1, a primary inductance 10 isconnected at one end to an antenna 1, and at its other end is connectedto a ground 2. A secondary inductance 12 has one ofits endsgrounded at16. A variable condenser 14 is connected across the inductance 12 toform a tunable resonant circuit 20 which is magnetically coupled to theantenna circuit formed by the antenna 1, inductance l and ground 2.Another inductance 18 is magnetically coupled with the inductance 12 ofthe resonant circuit 20, and connected at one end inseries with aninductance 24 to ground at 26, and at the other end to an inputelectrode or grid 28 of a thermionic tube 40. A variable condenser 22 isconnected across the inductance 18 to form a tunable resonant circuit30. The inductance 24 38. tube might be used. The cathode 36 isconnected is magnetically coupled with the inductance 10.

The thermionic tube 40 is indicated as including, beside the inputelectrode 28, an anode 32, an auxiliary grid 34, a cathode 36; and aheater However, any suitable type of thermionic to ground at 46 througha resistor 42. The potential drop across the resistor 42 provides abiasing potential between the cathode 36 and the input electr0de28. Acondenser 44 connected across the resistor 42 forms a by-pass circuitfor high frequency currents. The variable condensers l4 and 22 have beenindicated as being controlled by a common controlling element 50, and ina preferred form of the invention would be so used. 1

With particular reference to Fig. 2, like reference numerals in Figs. 1and 2 indicate similar parts. The principal difference between the twocircuits resides in the fact that the inductance 24 in Fig. 1 has beenreplaced by a condenser 48 which performs a similar function, and isconnected between the high potential or ungrounded ends of inductances18 and 12. One end of the inductance 18 is, of course, connecteddirectly to ground 26 instead of to ground through the inductance 24.

It is apparent to one skilled in the art that the purpose of a selectivecircuit, and particularly one of the type disclosed, is to tend toeliminatesignals other than the one that it is desired to receive, andto allow the signal that is to be received to pass without undueattenuation. In the circuit disclosed in Fig. 1, one element of theselection is brought about by the action of the resonant circuit 20.That is, since that circuit is tunable to resonance at a particularfrequency, and is coupled with the signal input circuit formed by theantenna 1, inductance and ground 2, the maximum current flow around theresonant circuit will be caused by the signal of the frequency to whichthe resonant circuit is tuned. Since the resonant circuit is tuned toresonance at the same frequency by the variable condenser 22, and iscoupled to the resonant circuit 20, a second element of selection isobtained through the action of the resonant circuit 30, and itscooperative action with the resonant circuit 20. However, since there iscoupling between the inductance l0 and the inductance 12, and alsocoupling between the inductance l2 and the inductance 18 there will alsobe some coupling between the inductance 10 and the inductance 18.

The direct coupling between the inductance 10 and the inductance 18somewhat impairs the selective action of the circuit as a whole, becausethe intermediate selective action of the resonant circuit 20 iseliminated or reduced. It has been found that the selectivity of thecircuit may consequently be improved by connecting an inductance 24 inseries with an inductance 18 and providing the proper amount of couplingbetween the inductance 24 and the inductance 10 in a proper phaserelation. It is believed that the voltage induced in the inductance 34tends to neutralize or buck the voltage induced in the inductance 18 bythe -field of inductance 10. The required amount of coupling between.the inductances 10 and 24 may be obtained by varying the number of turnsof the inductance 24, and the distance between inductances 10 and 24 ortheir relative angular displacement. A correct phase relation betweenthe voltage induced in the inductance 24 and that in the inductance 18may be determined by reversal of connection to the inductance 24 or thereversal of the direction of winding of the inductance 24. The resultantvoltage developed between the input electrode 28 and the ground 26 isthe voltage which is applied to the receiver or utilization device foramplification or utilization.

The coupling between the inductances l0 and 12, and 12 and 18 andbetween the inductance 10 and inductance 18, and the selective action ofthe resonant circuits 20 and 30 in Fig. 2 are similar to those ofFig. 1. However, the condenser connected between the high potential endsof inductances l2 and 18 serves to improve the selective action of thecircuit in a manner similar to that of the inductance 24 in Fig. 1. Itis believed that the potential applied to the high potential end of theinductance 18 from the high potential end of the inductance 20, in thismodification, serves to neutralize or buck the voltage induced in theinductance 18 by the field of the inductance 10. The potential thusapplied to the circuit of the inductance 18 may be varied to determinethe most desirable value by varying the size of the condenser 48. Thephase relation in this case is fixed, but could be changed by connectingthe condenser &8 to the low potential end of the inductance instead ofthe high potential end as disclosed.

While I have indicated and described several systems for carrying myinvention into effect, it will be apparent to one skilled in the artthat my invention is by no means limited to the particular organizationsshown and described, but that many modifications may be made withoutdeparting from the scope of my invention as set forth in the appendedclaims.

What is claimed is as follows:

1. A preselector circuit for a signal selecting device, comprising incombination, a plurality of magnetically coupled tunable resonantcircuits, an inductance magnetically coupled with one of said resonantcircuits, a second inductance connected to another one of said resonantcircuits and coupled with the first mentioned inductance in a manner toreduce coupling between said first inductance and the last namedresonant circuit.

2. A preselector circuit for a signal selecting device, comprising incombination, a plurality of magnetically coupled tunable resonantcircuits, an inductance magnetically coupled with one of said resonantcircuits, and a capacity for coupling said one circuit with a second ofsaid tunable circuits to minimize coupling between said inductance andsaid second circuit.

3. A preselector circuit for a signal selecting device, comprising incombination, a plurality of magnetically coupled tunable resonantcircuits, an inductance magnetically coupled with at least two of saidresonant circuits, and means for neutralizing the coupling between saidinductance and one of said two resonant circuits.

4;. A selective signal input circuit for a radio receiving device,including in combination, an antenna circuit, a tunable resonant circuitcoupled with said antenna circuit, a second tunable resonant circuitmagnetically coupled with the first mentioned resonant circuit, saidantenna circuit and said second circuit being normally coupled, andmeans coupling said second resonant circuit with said antenna circuitand tending to neutralize the normal coupling therebetween.

5. In a selective circuit for a radio device, the combination including,a primary circuit, a secondary circuit, a third circuit coupled withsaid secondary circuit and said primary circuit, and means coupling saidthird circuit to said primary circuit in a sense opposed to the firstcoupling therebetween for improving the operation of said selectivecircuit.

6. In a radio receiver, a pre-selector network comprising a pair ofcascaded resonant oscillation circuits, said circuits being magneticallycoupled, means for similarly tuning said coupled circuits to a commonfrequency, a source of signal energy magnetically coupled to each ofsaid coupled circuits, and means connected to the second of said coupledcircuits, electrically associated with at least a portion of the networkpreceding it in such a manner that the magnetic coupling between saidsource and said second circuit is decreased.

7. In a radio receiver, a pre-selector network comprising a pair ofcascaded resonant oscillation circuits, said circuits being magneticallycoupled, means for similarly tuning said coupled circuits to a commonfrequency, a source of signal energy magnetically coupled to each ofsaid coupled circuits, and means including a condenser connected to thesecond of said coupled circuits, electrically associated with at least aportion of the network preceding it in such a manner that the magneticcoupling between said source and said second circuit is decreased.

8. In a radio receiver, a pre-selector network comprising a pair ofcascaded resonant oscillation circuits, said circuits being magneticallycoupled, means for similarly tuning said coupled circuits to a commonfrequency, a source of signal energy magnetically coupled to each ofsaid coupled circuits, and means including a coil, connected to thesecond or" said coupled circuits, electrically associated with at leasta portion of the network preceding it in such a manner that the magneticcoupling between said source and said second circuit is decreased.

9. In combination, a signal selector network comprising at least twocascaded resonant circuits tuned to a common signal frequency, saidcircuits being reactively coupled, a source of signal energy reactivclycoupled to each of said coupled circuits, and reactive means, connectedto the second of said coupled circuits, electrically associated with atleast a portion of the network preceding the second circuit in such amanner that the effect of the reactive coupling between said source andsaid second circuit on the selectivity of the network is decreased.

10. In a network as in claim 9, said reactive couplings being magnetic,and said reactive means consisting of a condenser connected between thehigh signal potential sides of the two coupled circuits.

HOMER J. LOFTIS.

